Sound when my buddies send me a file?

Hi, can anybody kindly tell me how to set pidgin to produce a sound
when a buddy sends me a file and requests me to accept it? (Usually
there should be a pop-up window asking me if I want to accept that
request). I have looked into Tool--->Preference--->Sound but I did not
find any event corresponding to "sending file".... I am using pidgin
2.6.3 under linux. Thank you:)
Peng